I did 100% completion on survival difficulty. (Yes it is my second run)
I saw the message thunderclap unlocked when I achieved 100%.
Then I died trying to return to the camp.
The last checkpoint was right next to the camp so I checked my weapon selection.
And amazingly, thunderclap was not there.
Since I ignored the Baba Yaga dlc, I thought it was because of that.
So I completed the dlc.
And still I cannot see the thunderclap in camp.

Any thoughts or ideas about this?

I would be quite upset if the only solution is re-complete the game.